1. Field of the Invention
The present invention relates to a wavelength division multiplexed passive optical network, and, more particularly, to temperature monitoring and wavelength compensation within the WDM system.
2. Description of the Related Art
Typical optical subscriber networks use a double star structure in order to minimize the length of an optical line needed to connect the network nodes together. From an optical line terminal (OLT) to a remote node (RN) installed at an area adjacent to the subscriber units, the OLT and RNs are connected via a single strand of optical fiber. The remote nodes, are similarly connected to each associated subscriber unit through a separate optical fiber.
In a wavelength division multiplexing (WDM) systems the subscriber units use or are assigned different wavelengths or channels, each subscriber unit may be referred to by their associated wavelength or channel number. OLTs and the remote nodes must include a multiplexer unit (MUX) for multiplexing individual optical signals and a demultiplexer (DEMUX) for demultiplexing the multiplexed optical signals. A waveguide grating router (WGR) is typically used as for such multiplexer/demultiplexer operations
As the remote nodes of a WDM optical network installed at a site adjacent to the subscriber units may be distributed over large distances, the remote nodes may be affected by temperature change caused by changes of season or even by whether it is day and night differently than other RNs or the OLT. The temperature range that a remote node must operate over has been established by system specifications of at least 120 degrees Centigrade (deg. C.), i.e., from −40 to +80 deg. C., and a maximum change rate of temperature of 1 deg. C./min.
The WGR used as a multiplexer/demultiplexer, as described above, has a rate of change of wavelength with respect to temperature that is determined based on the materials from which the WGR is made. For example, when the WGR is made from general semiconductor materials, the rate of change of wavelength with respect to temperature is about 0.1 nm/deg. C. On the other hand, when the WGR is made from silicon dioxide (SiO2) the rate of change of wavelength with respect to temperature is about 0.015 nm/deg. C. See for example, ‘A Wavelength-Matching Scheme for Multiwavelength optical Links and Networks Using Grating Demultiplexers’, F. Tong, et. al, Journal of IEEE Photon. Tchnol., Lett., seventh volume, pp. 688-690, (1995).
Consequently, the wavelength of a WDM light source for downstream transmission and the received wavelength of the WGR at the remote node (or the wavelength of the WGR in the OLT and the wavelength of the WGR in the remote node) may not coincide with each other, resulting in an output loss on channels transmitted. Crosstalk between adjacent channels is further increased as the nominal wavelength of one channel approaches the nominal wavelength of an adjacent channel. Hence, the transmission performance of a system begins to deteriorate as the temperature in the OLT and/or the remote nodes changes.
In order to prevent the transmission performance from being deteriorated, wavelength tracking methods have been proposed that can equalize the wavelength of the WDM light source(s) for downstream transmission with the wavelength of the WGR in the remote node as changes occur according to temperature of the site. Such a wavelength tracking methods are well-known in the art and need not be discussed in detail herein. See, for example, ‘Fiber-Grating Sensor for Wavelength Tracking in Single-Fiber WDM Access PON's’, Randy Giles and Song Jiang, Journal of IEEE Photon. Tchnol., Lett., ninth volume, pp. 523-525, (1997); ‘Wavelength Tracking of a Remote WDM Router in a Passive Optical Network’, D. Mayweather, et. al, Journal of IEEE Photon. Tchnol., Lett., pp. 1238-1240 eighth volume (1996); and ‘Demonstration of a 12×155 Mb/s WDM PON Under Outside Plant Temperature Conditions’, R. Monnard, et. al, Journal of IEEE Photon. Tchnol., Lett., pp. 1655-1657, ninth volume (1997).
Conventional wavelength tracking method use a monitor channel dedicated only for wavelength tracking, a dedicated optical fiber for providing the monitor channel to a central office and an optical fiber diffraction grating. In one method, a measure of the difference between the wavelength of the WDM light source at the OLT and the wavelength of the WGR in the remote node is determined and the temperature of the WDM light source is adjusted so that the wavelength of the light source coincides with the wavelength of the WGR in the remote terminal.
Another method of equalizing the wavelength of the WDM light source for downstream transmission in the OLT is described in Korea Patent Application No. 1999-35226 (filed on Aug. 24, 1999, KAIST), entitled “An Apparatus and a Method for Tracking a Wavelength in a Spectrum-Sliced WDM Passive Optical Network”. The wavelength tracking method of equalizing the wavelength of the WGR in the OLT with the wavelength of the WGR in the remote node proposed in Korea Patent Application No. 1999-35226 is shown in FIG. 1 for the upstream transmission in a WDM passive optical network. A similar operation is proposed for downstream transmission and need not be described in detail herein.
The passive optical network shown includes a central office 100, a remote node 200, and an upstream line for transmitting an upstream signal from remote node 200 to central office 100.
Remote node 200 includes multiplexer 201 represented by a WGR, a temperature controller 202 that provides a current to a thermoelectric cooler 220, that is used to control or adjust the temperature of multiplexer 201.
Central office 100 includes an erbium-doped fiber amplifier 102 for amplifying an upstream signal, demultiplexer 101, represented WGR, and a wavelength tracking apparatus 110. The wavelength tracking apparatus 110 adjusts a current provided in a thermoelectric cooler 120 in order to control and adjust the temperature of the demultiplexer 101.
Wavelength tracking apparatus 110 includes optical coupler 104, photodetector 111, a plurality of optical couplers 105 and 106, a plurality of photodetectors 115 and 116, a plurality of diodes 117 and 118, an amplifier 119, a differential amplifier 113, and a control circuit 112.
The optical coupler 104 is located between the erbium-doped fiber amplifier 102 in an upstream line and the demultiplexer 101 and provides optical power passing through the upstream line to the photo detector 111. In the embodiment of the invention shown the optical coupler 104 distributes the received optical signal at the rate of 99:1. One skilled in the art would recognize that the coupler may distribute the optical power in other ratios without altering the scope of the invention. The photodetector 111 receives the optical signal distributed from the optical coupler 104 and generates and outputs a voltage proportional to the intensity of optical power.
The optical couplers 105 and 106 are connected to channels of output terminals of the demultiplexer 101 and distribute optical power outputted from the demultiplexer 101 to optical power meters 107 and 108 and the photodetectors 115 and 116, respectively. The photodetectors 115 and 116 generate and output a voltage proportional to the intensity of the optical power received from the optical couplers 105 and 106.
The photodetectors 115 and 116 are connected to the amplifier 119 through the diodes 117 and 118, respectively. The highest voltage of the voltages generated by photodetectors 115 and 116 is provided to the amplifier 119 through the diode 115 or 116 and is then amplified by the amplifier 119. The differential amplifier 113 has an inverting terminal connected to an output terminal of the amplifier 119 and a non-inverting terminal connected to an output terminal of the photodetector 111, and the differential amplifier 113 amplifies and outputs the difference between two input signals. The control circuit 112 outputs a current to the thermoelectric cooler 120 for adjusting temperature of demultiplexer 101. The level of current is determined based on the size and change of the signal outputted from the differential amplifier 113.
In the system described above, an optical signal in each channel is split, e.g., 10%/90%, and the intensities of the 10% signals are detected by the photodetectors 115 and 116, and the detected intensities are added to each other. Then, the intensity obtained through the addition is compared with an intensity of an optical signal before passing through the WGR 101. When the two intensities are different from each other due to abnormality of any one channel or the photodetectors, the control circuit 112 determines that a wavelength is shifted due to the change of temperature and performs an operation such as an adjustment of temperature occurs.
Since intensities of the optical signals passing through respective channels must be added to each other, couplers and photodetectors must be provided to respective channels in order to split the optical signals. Therefore, the cost of a system increases. Hence, there is need for a low cost system and method for monitoring wavelength shift and adjusting temperature to compensate for such shift.